As I said it has a steep learning curve and documentation is pretty much the nixpkgs repo itself (well after understanding the basics of Nix and NixOS at least, with the combination of the https://nixos.wiki mostly IMO). It also takes some time to get used to the quirks of NixOS (and understanding the necessary practical design decisions of these quirks).

But I have nowadays seldom trouble with switching the generations (i.e. nixos-rebuild switch), unless you're updating flake inputs or (legacy) channels (where e.g. a new kernel might be used). In that case it makes sense to reboot into the new configuration. Also, obviously that can lead to short down-times (including just restarting a systemd service, if a service has changed in between the generations), if that is unacceptable, there obviously needs to be a more sophisticated solution, like kubernetes via e.g. kubnix. I'm not sure how much of that can be achieved with Ansible, as I haven't used it that much because I disliked the "programming" capabilities of the Ansible yaml syntax (which feels kinda hacky IMHO).

But apart from NixOS, one can also just use Nix on a different system to e.g. deploy or create docker images (which can be really compact, as only the necessary dependencies for a package is packaged) that in turn could e.g. be managed with Ansible or something...